MINERAL PROPERTIES: CLEAVAGE
(Including Fracture and Parting)

Cleavage, fracture, and parting all have
to do with the positioning of atoms in a mineral and how it breaks when put under stress.
(These three properties are listed on the same page due to their comparability, but are
each individually discussed).

In mineral terms, cleavage describes how a crystal breaks when subject to stress on a
particular plane. If part of a crystal breaks due to stress and the broken piece retains a
smooth plane or crystal shape, the mineral has cleavage. A mineral that never produces any
crystallized fragments when broken off has no cleavage.

Cleavage is often measured by three factors:1) Quality of Cleavage2) Number of Sides Exhibiting Cleavage3) Cleavage Habit

Quality of Cleavage:

Quality of cleavage can be categorized into five qualities:

Perfect

Good

Poor

Indiscernible (Indistinct)

None

Minerals with perfect cleavage will cleave without leaving any rough surfaces; a
full, smooth plane is formed where the crystal broke. Minerals with good cleavage
also leave smooth surfaces, but often leave over minor residual rough surfaces.
On minerals with poor
cleavage, the smooth crystal edge is not very visible, since the rough surface is
dominant. If a mineral exhibits cleavage, but it so poor that it is hardly
noticeable, it has "indiscernible" cleavage. Minerals with no cleavage
never exhibit any cleavage, thus broken surfaces are fractured and rough.

Categorization of cleavage qualities is not scientifically
affirmed. The above categorization is used by most mineral references, but some
guides
categorize cleavage in three or four groups, and may give them different names,
such as "excellent" and "distinct".

Number of Sides Exhibiting Cleavage:

Many minerals exhibit cleavage only on one side, and some may exhibit different
quality cleavage on different crystal sides. The following criteria may be expected when
analyzing the cleavage of any particular mineral:

One Direction

Two Directions

Three Directions

All Directions

These identify how many "directions", or planes, the crystal is exhibiting the cleavage on. Each direction
signifies the two opposite sides of a
three-dimensional figure, (since opposite sides will always exhibit the same cleavage properties). If
a mineral has cleavage in three directions, then every side of the mineral has cleavage
(i.e. length, width, and height).
If a mineral occurs in modified crystals with more than six sides (i.e. an octahedron)
and exhibits cleavage on all the sides, than it has cleavage in "all
directions".

Combining the cleavage level together with the number
of sides will measure the cleavage of a mineral. For example, if a mineral has Good
Cleavage, Two Directions, this means that it has
good cleavage on four out of six sides (while the other two sides exhibit no
cleavage). If a mineral has Perfect Cleavage, One Direction; Poor Cleavage, Two
Directions, this means that the mineral has perfect cleavage on two sides, and poor
cleavage on the other four.

In this guide, cleavage quality is measured in numbers, then the amount of sides, separated by a comma. 1 is perfect cleavage, 2 is good cleavage, and 3 is poor cleavage. If the cleavage of a mineral is
written as 1,2 the mineral has perfect cleavage in two directions. If all sides of mineral have the same cleavage, and the mineral
often occurs in modified crystals with more than six sides, than All Sides is
written instead of a number. If a mineral exhibits different cleavage on different crystal
planes, there will be two cleavage indicators separated by a semi-colon (;). For example,
if the cleavage of a mineral is written as 1,2;3,1, than it has perfect cleavage in
two directions, and poor cleavage in one other direction. If a mineral exhibits indistinct or no
cleavage, Indiscernible or None is written in the cleavage field.

Cleavage Habit:

Different habits of cleavage exist on different minerals, depending on
their mode of crystallization.
These forms of cleavage are:

Basal cleavage:Cleavage exhibited on a horizontal plane of
the mineral by way of its base. Minerals with basal cleavage can sometimes be
"peeled".
An example of basal cleavage are the mica minerals.

Cubic cleavage:Cleavage exhibited on minerals of the isometriccrystal system that
are crystallized as cubes. In this method of cleavage, small cubes
evenly break off of an existing cube.
An example is Galena.

Octahedral cleavage:Cleavage exhibited on minerals of the isometriccrystal system that
are crystallized as octahedrons. In this method of cleavage,
flat, triangular "wedges" peel off of an existing octahedron.
An example is Fluorite.

Prismatic cleavage:Cleavage exhibited on some prismatic
minerals in which a crystal cleaves as thin, vertical, prismatic crystals off
of the original prism.
An example is Aegirine.

Parting is characteristically similar to cleavage. It is
easily confused with cleavage, and it may be present on minerals that do not exhibit any
cleavage. There are two causes of parting:

Two separate pressures pushed toward the center of a crystal
after its formation, causing the crystal interior to evenly dislodge on a flat, smooth
plane.

Twinned crystals
that separated from one another, leaving a flat, smooth plane.

With enough perception, a distinction can be made between
parting and cleavage. If fracture marks
are present on a crystal in addition to a cleaved
plane, the "cleaved" surface is usually the result of parting, not cleavage. An
outline of a crystal etched in a mineral is also the result of parting, in the form of
twinned crystals that separated.

In general, one need not worry about confusing parting with
cleavage. Parting is uncommon, and it can usually be determined by the distinguishing
characteristics mentioned above.

Fracture is the characteristic mark left when a mineral
chips or breaks. Cleavage and fracture differ in that cleavage is the break of a crystal face
where a new face (resulting in a smooth plane) is formed, whereas fracture is the
"chipping" shape of a mineral. All minerals exhibit a fracture, even those that
exhibit cleavage. If a mineral with cleavage is chipped a certain way, it will fracture
rather than cleave.

There are several terms to describe the various mineral
fractures:

Conchoidal - Fracture resembling a semicircular
shell, with a smooth, curved surface. An example of conchoidal fracture can be
seen in broken glass. (This fracture is also known as "shelly"
in some reference guides.)

Uneven - Fracture that leaves a rough or
irregular surface.

Hackly - Fracture that resembles broken
metal, with rough, jagged, points. True metals
exhibit this fracture. (This fracture is also known as "jagged".)

Some references may describe additional fractures not
mentioned above, but those terms are either synonymous or simply used as a verbal
depiction of the authors inference.

Almost all minerals have a characteristic fracture. Some
minerals of the same species may exhibit a different fracture, but this is rare.

How to use cleavage, parting, and fracture as an identification mark

A specimen need not be broken to check its fracture habit
or cleavage. Rather, it should be checked for areas of stress where it could have broken
or chipped. Fracture marks are rarely present on minerals with good or excellent cleavage.
Minerals with poor cleavage will fracture more often than those with good or perfect
cleavage.

How to testing using cleavage, parting, and fracture

Observe the mineral to see if it has any cleaved surfaces
or fractured edges. If it has cleaved surfaces, the quality of the smoothness on the
surface should be noted. If there are no visible cleaved surfaces, it does not mean the
mineral does not exhibit cleavage. It is possible that particular specimen did not cleave.
On such a specimen, it is only possible to check cleavage by chipping off a piece. This
should be done gently and carefully in an area which will not degrade its value. If there
is a noticeable fracture on the mineral, it is an likely indication that the mineral probably
has poor cleavage or none at all.

Careful observation and experience should also be able to distinguish between a cleaved
crystal and a crystal that parted.